NO118466B - - Google Patents

Description

Subscription television system.

The present invention relates to subscription television systems, in which a television signal is broadcast in coded form so that it can only be used in subscription receivers equipped with decoding devices which are controlled in dependence on the coding scheme used in the transmitter.

The present invention relates to a subscription television system for use

ten in a transmitter or in a receiver and comprising a coding (decoding) device with a plurality of working states,

each of which creates a different way of working in the transmitter (receiver), a code (decode) producing device

comprising a source for randomly occurring coding or decoding signal components associated with a plurality of adjustable coding (decoding) determining elements whose momentary setting determines at least partial application of said signal components

on a control device, which in turn produces a control signal that represents

rer a predetermined coding scheme, and a device for utilizing the control signal to switch the coding (decoding) device between its operating states to vary the mode of operation of the transmitter (receiver).

The system is mainly characterized by

net by a feedback between an out-

time and an input of the control device via a switching device, from which output the switching device is impressed with a modification signal that stands in relation

to the aforementioned predetermined secret code scheme, and which is utilized to, from time to time, reset at least part of the control device to a predetermined reference state in order to thereby reset the encoding (decoding) device in a manner determined by the predetermined secret code scheme modified by the switching device.

A large number of different subscription television systems have been proposed

more, in which a key signal or coding signal, which contains the coding scheme information in unordered or cam-

several forms, is transmitted in the form of a modulation component for the coded television signal itself or over an additional cable

needle. The wireless key signal or coding signal is then transformed in the receiver in a way known only to the subscribers concerned, and the transforming key signal is then used to influence suitable decoding devices.

ter for decoding the coded television signal. Distortion and transformation or recovery of the key signal can e.g. is achieved by means of a plurality of code-determining elements, such as e.g. flip-flops or switches in a transposition mechanism that can be used to change some of the signal's characteristic properties or to direct different parts of the signal to different input circuits in the coding appa-

rated. The particular setting of the transposition mechanism or the code-determining elements, which are needed for a pre-

intelligible reproduction of a specific television programme, of course only the relevant subscribers are notified, whereby it is achieved that the payment of the subscription fee can be secured in the desired way.

In previous systems, a combination of coding signal pulses are emitted, which individually have a predetermined identification characteristic, such as e.g. frequency, to subscription recipients during each subframe interval together with the composite video signal. These pulses, which preferably come in an arbitrary order and appear arbitrarily within each combination, are derived from the composite television signal on the receiver side and separated from each other by means of suitable filters, in order to be fed to a transposition mechanism via specific input circuits. This mechanism can use a group of selectors or switches, which serve as code-determining elements for optionally establishing a plurality of connections between the input circuits and a plurality of output circuits, which in turn are connected to different input circuits in a multistable control device, comprising a plurality of bistable multivibrators. By means of this device, the code signal pulses can be supplied to the input circuits of the control device in a prescribed order for transferring this device from one to another of its stable operating states. Changes occur in the working mode of the television system by varying the relative time sequence of the video and synchronization components of the television signal, and the scheme for the working mode change is determined by the way in which the control device works during each sub-frame interval.

The coding technique of the previously known devices is very efficient, and allows the use of a wireless coding signal while achieving a significant degree of secrecy. However, it may be desirable to further improve the secrecy, to further reduce or completely eliminate even a remote possibility that the person concerned will be able to receive the television program.

In the previously known systems, the common or overall pattern for the various code-determining elements, which are used at each receiver, determines, at least in part, the operation of the decoding apparatus. If all elements are set correctly, decoding is achieved. On the other hand, if the code-determining elements are all incorrectly set, it results in a completely distorted and disordered image. However, there is a remote possibility that an unauthorized person in random and rare cases, by using a trial and error method when manipulating the code-determining elements, can achieve a partial arrangement of the image as a result of some of the code-determining elements being correct tuned in. He can then concentrate his efforts on the elements that are not correctly set, and as each individual element is set correctly, it will be possible to determine and guide the approach to a correct setting by observing and evaluating faint visible traces in form of progressive minor improvements in certain critical details of the image reproduction. The present invention provides a device to prevent or reduce the possibility of a partial decoding, if some, but not all code-determining elements are correctly set. In other words, if an unauthorized person tries to achieve a correct combination or pattern by trial and error methods, considerably fewer and even fainter visible traces will be observed on the receiver screen as he approaches the correct combination, so that in reality any possibility of unauthorized reception of the television program is eliminated.

This desired secrecy is achieved according to the present invention by deriving synchronous modifying signals from the encoding device in the transmitter and the decoding device in the subscription receiver, which signals are in turn used for further control of the device's function. If all the code-determining elements of a subscription receiver in such a system are not precisely tuned to achieve the correct operation of the decoding apparatus, the modifying signal is incorrect; this results in a completely erroneous or asynchronous operation of the decoder.

It is accordingly an object of the present invention to provide a new and improved subscription television system, in which a television signal is coded in a very complicated manner.

It is another object of the invention to provide an improved subscription television system, which uses a wireless coding signal, and where coding and decoding are provided in such a way that the possibility of unauthorized decoding is in fact completely eliminated.

In the previously known systems, which are discussed in the above, the used control device, which preferably has the form of one or more bistable multi-vibrators, can possibly be affected by noise or other extraneous signals with the resulting asynchronous function of the transmitter and the receiver. This has previously been avoided by sending reset or reset pulses, which are applied directly to the bistable multi-vibrators to reset them to their initial state or by using specific pulses with each coding combination, which are passed through the transposition mechanism to specific input circuits in the multi-vibrators with the same effect as the reset pulses, namely to set the multi-vibrators to their initial state. When applying the present invention, it has been found that no specific reset pulses are needed, either as part of the coding combination or sent out at any other time. This represents a significant improvement, compared to previous systems, as it is no longer necessary to provide either separate reset circuits for the bistable multivibrators or to limit the choice or use of coding combinations to those comprising predetermined reset pulses, which serve to set the the control device to a certain output state.

It is therefore a further aim of the present invention to provide a subscription television system, in which all possible coding signal combinations can be used arbitrarily and without limitation, while stable synchronization is maintained between the transmitter and the subscription receiver devices.

A subscription television system according to the present invention comprises an encoding apparatus and a decoding apparatus for varying the working method of the system in accordance with a predetermined encoding scheme. Means are provided for the derivation of a modifying signal from the coding or decoding apparatus. The system further comprises devices, connected to the device for deriving the modifying signal and to the coding or decoding apparatus for using the modifying signal to change the working state of the coding or decoding apparatus for changing the predetermined coding scheme.

According to a preferred embodiment of the invention, the coding scheme is changed by adding the modification signal, which preferably represents an arbitrary or random scheme to a switching device such as e.g. a coincidence or gate circuit, which leads code signal components to the encoding apparatus, respectively the decoding apparatus. With this device, the state of the switching device will be varied by the modifying signal, so that only certain coding signal components are supplied to the coding or decoding apparatus in accordance with the coding scheme for changing the system's working mode.

When coding a subscription television signal using a combination of coding signal components, it is sometimes desirable to additionally use a series of periodically recurring signal components in order to increase the degree of disorder. In order to achieve maximum coding, it is desirable that the periodically recurring components do not occur at a specific time or in specific phase relationships to the coding signal components. If, however, these two somewhat independent groups of signal components are supplied to the same coding apparatus, ambiguity or instability may occur as a result of a possible simultaneous supply of a periodically recurring signal component and a coding signal component.

It is therefore an object of the invention to provide a subscription television transmitter, in which a coding device is jointly affected by periodically recurring signal components and coding signal components which are synchronized in relation to each other so that the components do not interfere with each other.

When a multi-stable counting device, such as a blocking oscillator is used to generate the periodically recurring signal components, it is desirable to generate a series of arbitrarily occurring switching or reset pulses, which are used to set the counting device in a predetermined reference state from time to time. This precaution ensures synchronization between the transmitter and subscription receiver devices. If, however, one of the reset pulses happens to occur immediately after the counting device's operating cycle has ended or during one of the earlier steps in the cycle, unstable operation could occur as a result of many counting devices' functional limitations.

It is therefore a further object of the invention to provide a subscription television transmitter with a multi-step counting device, which is reset by means of randomly occurring reset pulses, only during certain of the various steps in the counting cycle of the device.

The features of the invention, which are considered to be new, are indicated specifically in the claims. The invention, as well as further purposes and advantages of it, will however be best understood from the following description in connection with the drawings, where

fig. 1 is a block diagram for a subscription television transmitter according to an embodiment of the invention.

Fig. 2 is a detailed connection diagram for part of the transmitter in fig. 1. Fig. 3 is a detailed connection diagram for another part of the transmitter of fig. 1. Fig. 4 and 5 together with fig. 5 placed immediately below fig. 4 a group of curves, which serve to explain the operation of the transmitter in fig. 1. Fig. 6 is a block diagram of a subscription television receiver according to an embodiment of the invention, and intended to cooperate with the transmitter of fig. 1. Fig. 7 is a block diagram of a section of the transmitter in fig. 1 and the receiver in fig. 6 with circuit connections arranged in accordance with another embodiment of the invention. Fig. 8 is a block diagram of a section of the transmitter of fig. 1 and the receiver in fig. 6 with the circuit connections arranged in accordance with a further embodiment of the invention. Fig. 9 is a block diagram similar to fig. 7 and 8, and shows a further embodiment of the invention. Fig. 10 is a corresponding block diagram of a further embodiment of the invention. Fig. 11 is a block diagram of a part of a subscription television transmitter, made in accordance with a further embodiment of the invention. Fig. 12 graphically shows different waveforms and serves to explain the operation of the system according to fig. 11, and

fig. 13 is a block diagram of part of a subscription transmitter, made in accordance with a final embodiment of the invention.

The transmitter in fig. 1 comprises an image converting device 10, which can be an iconoscope, an image orthoicon or another known device for producing a video signal, which represents the image to be transmitted. The output channels for the device 10 are connected via a video amplifier 11 and a coding device 12 to the input channels in a mixing amplifier 13. The coding device 12 can be designed in a similar way as according to previously known systems. It may comprise a beam deflection tube with a pair of output circuits, which can optionally be connected in the video channel when the electron beam is deflected from one to the other by two segment anodes, which are connected to these output circuits. One of these output circuits comprises a time delay element, so that time positions of the video components in relation to the synchronization components of the transmitted signal vary when the beam in the deflection tube is deflected between the tube's anodes. This deflection effect is achieved by means of a beam deflection signal, which is supplied to the coding apparatus 12, as will be explained in the following. Such intermittent variations in the relative timing of the video components and the synchronization components result in an efficient coding of the television signal, as ordinary television receivers, which are not provided with suitable decoding equipment, need a constant or non-variable timing of the video components in relation to the synchronization components for producing an intelligible image reproduction.

The coding apparatus 12 has at least two stable working or operating states, each of which determines a different mode of operation in the transmitter. In the first operating state, the encoder 12 connects the video channel from the amplifier 11 to the mixer 13, without any significant time delay, and in this state the function of the transmitter is as usual, especially with regard to the time relationship between the video and synchronization components of the transmitted signal. In its second state, the encoder introduces a time delay in the video channel, and the transmitter operates in a way that deviates from the normal one, in that the video components and the synchronization components in the transmitted signal have an abnormal time relationship to each other.

The mixer 13 is connected through a level fixer 14 to a carrier wave generator and modulator 15, which in turn is connected to an antenna 16. The transmitter also comprises a synchronization signal generator 19, which delivers partial image and line synchronization components and thus connected blanking components to the mixing amplifier 13, through suitable current circuits, which are schematically indicated here as a single conductor 20. The generator 19 further supplies partial image and line drive pulses to a partial image scanning system 17 and to a line scanning system 18. The output channels for the scanning systems 17 and 18 are connected to partial image and the line deflection elements (not shown), which are connected to the image conversion device 10.

The synchronization signal generator 19 further supplies line drive pulses to a conventional blocking oscillator 30, the output channels of which are connected to the input channels of a conventional bistable multi-vibrator 57, so that the multi-vibrator switches from one to the other of its two stable operating states depending on successive applied pulses. As an example, it can be assumed that the blocking oscillator 30 is set to a count ratio of 5:1, although other count ratios can also be used. The multi-vibrator 57 is connected via a buffer amplifier 59 to the input channels of another bistable multi-vibrator 58, the output channels of which are connected to the coding apparatus 12. As explained in more detail below in connection with fig. 2, the multi-vibrators 57 and 58 form parts of a control device which operates in dependence on supplied pulses from the blocking oscillator 30 to cause switching of the coding apparatus 12 between its working states for coding the television signal in accordance with a predetermined coding scheme.

The generator 19 finally supplies sub-image drive pulses to an input circuit for a coding signal generator 29 and line drive pulses to another input circuit for the generator. The generator 29 has an output circuit connected to the synchronization signal generator 19 over the conductor 28, and another output circuit over a conductor 66 connected to an input circuit of the mixing amplifier 13 and over the conductor 68 to the input circuit of a series of filter-rectifiers 21-26. The coding signal generator 29 is arranged to emit a combination of coding signal components, which individually have a predetermined identification characteristic, such as e.g. frequency, and together determine a coding scheme in accordance with their appearance and sequence in the combination. A suitable coding signal generator can be made as previously known, and the generator 29 is therefore only shown schematically to avoid unnecessary complication of the description and drawings.

In the illustrated embodiment, the encoding signal generated during each subframe rewind interval may comprise a number of 5 or 6 pulses of any of six different signal frequencies f,-f„, preferably in any order and generated at any time within interval, and these pulses are generated individually between successive line drive pulses, which are superimposed on the vertical quench level. Each filter rectifier 21-26 can be selected for one of the different signal pulse frequencies f,-f„ to facilitate the separation of the components from each other for optional input to a number of input circuits of a transposition mechanism 27. This mechanism, which is not tuned in accordance with a prior particular setting pattern, is arranged for optional connection of any of the six filter-rectifiers 21-26 with any of the five output circuits or conductors 31-35, and may comprise a group of switches or switches, which the code- ning-determining elements.

The conductors 31—35 are connected to the respective normally blocked gate circuits 36—40, which are likewise supplied with line drive pulses from the synchronization signal generator 19. The output circuits for the gate circuits 36 and 38 are connected via conductors 41 and 43 to the bistable multi-vibrators 58, while the output circuits for the gates 37 and 39 over conductors 42 and 44 respectively are connected to the bistable multivibrator 57.

According to this embodiment of the invention, the blocking oscillator 30 with a count ratio of 5:1 is likewise connected to another series or set of cascaded bistable multivibrators. More particularly, a bistable multi-vibrator 57' is likewise connected to the blocking oscillator 30, and the output circuits for this multi-vibrator are connected via a buffer amplifier 59' to another bistable multi-vibrator 58'. The output circuit of the multi-vibrator 58' is connected via a delay line 71 to the input circuit of a non-linear signal converter or coincidence circuit 46, which has another input circuit connected to the gate circuit 40 over the conductor 45. The output circuit of the coincidence stage 46 is connected with an input circuit for the blocking oscillator 30 across the conductor 47 for resetting, i.e. for setting the oscillator 30 in a reference state whenever a pulse occurs in the output circuit of the coincidence stage 46. As in the case of the multivibrators 57 and 58, the multivibrators 57' and 58' of pulses from the coding signal generator 29, which are transmitted to the gates 36-39 through the transposition mechanism 27. The output circuits of the gates 36 and 39 are connected via the respective conductors 41 and 44 to the bistable multi-vibrator 57' and the output circuits of the gates 37 and 38 are over the conductors 42, respectively 43, connected to the bistable multi-vibrator 58'.

Now the construction of the multi-vibrators 57 and 58 and the buffer amplifier 59, which is shown in detail in fig. 2, is described.

Considered together, these multi-vibrators have several successive functional steps, and are affected during this sequence by the periodic returning pulses, which are obtained from the blocking oscillator 30, and via a capacitor 103 are supplied to the control electrode 104 in an electron discharge device 105 and via a capacitor 107 to the control electrode 109 in an electron discharge device 110, the devices 105 and 110 being cross-connected to form a common bistable multi-vibrator circuit 57. The control electrode 112 in the buffer amplifier tube 116 is connected to the anode 106 in the device 105 via a capacitor 113, while the control electrode 112 also is connected to a source for a negative potential 115 across a resistor 114, which in combination with the capacitor 113 forms a differentiation circuit. The anode 117 in the discharge device 116 is connected to B+ through a load resistor 126 and through capacitors 118 and 119 to the control electrodes 122 and 123 in a pair of electron discharge devices, 120 and 124 respectively, these devices and their associated circuits forming the usual bistable multi -vibrator 58. The output conductor 44 from the gate circuit 39 is connected to the control electrodes 104 and 109 through capacitors 103 and 107, while the output conductor 42 from the gate circuit 37 is connected to the anode 108 in the device 110, and the output conductor 41 from the gate circuit 36 is connected to the control electrodes 122 and 123 through the capacitors 118 and 119 respectively, and the output conductor 43 from the gate circuit 38 is connected to the anode 125 in the device 124.

Reference must now be made to fig. 3, which illustrates in detail the multi-vibrators 57' and 58' and the buffer amplifier 59', from which figure it will be seen that, with the exception of the connections from the gate circuits 36-39, the circuit diagram is identical to that shown in fig. 2, as indicated by corresponding marked reference numbers. However, the input connections from the gate circuits are different so that the control signal produced in the output circuit for the multi-vibrator 58' must be distinctly different from that produced in the output circuit for the multi-vibrator 58. More particularly, the output conductor 44 from the gate circuit 39 is connected to the control electrodes 104' and 109' through the capacitors 103', respectively 107', while the output conductor 41 from the gate circuit 36 is connected to the anode 108' in the device 110'. The output conductor 42 from the gate circuit 37 is connected to the control electrodes 127' and 123' over the capacitors 118', 119' respectively, and the output conductor 43 from the gate circuit 38 is connected to the anode 125' in the device 124'.

Briefly stated, the multi-vibrators 57 and 58 together with the blocking oscillator 30 form a first control means for generating a first control signal with an amplitude characteristic, which varies between at least two predetermined values in accordance with a predetermined first secret coding scheme, which is determined in part by the distribution of the coding signal pulses and partly by switch setting patterns for the transposition mechanism 27. This first control signal represents secret coding information and is supplied to the coding apparatus 12 to cause it to be set between its working states in accordance with the first coding scheme. The multi-vibrators 57' and 58' and the delay line 71 in cooperation with the blocking oscillator 30 form another control means for generating another control signal with an amplitude characteristic, which varies between at least two predetermined values in accordance with another predetermined coding or control scheme, which is distinctly different from the first coding scheme and is likewise determined by the distribution of the coding signal pulses and the setting of the transposition mechanism. The output circuit of the gate 40 can be considered as a source of coding signal components, which are applied to the coincidence circuit 46. The second control signal, which represents the control information, serves as an opening signal for the coincidence circuit 46 so that only a certain one of the coding signal components should be applied to the blocking oscillator 30, which component occurs during the time intervals, in which the amplitude of the second control signal is at a predetermined amplitude level.

In order to simplify the detailed explanation of the invention, the operation of the described transmitter shall initially be considered without regard to the coding technique. The image converting device 10 produces video frequency components, which represent the image information to be transmitted, and these components are supplied after amplification in the video amplifier 11, through the coding device 12 to the mixer amplifier 13. The mixer further receives the usual line and field image synchronization pulses and reference pulses over the conductor 20 from the synchronization signal generator 19. The composite video signal, which is provided by means of the mixing amplifier 13, is set to a suitable background level in the level fixer 14, and is amplitude modulated on the image carrier wave in the device 15 to form a composite television signal. The modified video carrier wave is supplied to the antenna 16 for broadcast and transmission to subscription recipients. It will of course be understood that during the generation of the video frequency components, the sweep systems 17 and 18 are synchronized by the partial image and line drive pulses from the generator 19. As in any television broadcast, the associated audio information is modulated on a sound carrier wave and broadcast simultaneously; the sound system can be of a completely normal type or can possibly also include suitable sound coding devices.

Briefly, coding of the video portion of the broadcast is provided by the coding apparatus 12 under the influence of a control signal produced by the multi-vibrator 58, which bends the beam in the beam deflection tube back and forth between the two segment anodes in accordance with a secret coding scheme which is represented by control signal amplitude variations. As explained above, this influence of the coding device varies the transmitter's way of working by varying the time ratio between the video and synchronization components in the transmitted signal, whereby an effective coding or distortion of the image is achieved.

To produce a deflection control signal for the encoder 12 (disregarding for the moment the specific function and effect of the encoder signal pulses produced by the generator 29) line drive pulses are applied to the common 5:1 blocking oscillator 30, in which they are effectively frequency divided in the ratio 5:1, so that every fifth line drive pulse will be supplied to the bistable multi-vibrator 57. This multi-vibrator operates in the usual way, and switches between its operating states in dependence on successively supplied pulses from the oscillator 30 to supply a periodic square- wave signal to the buffer amplifier 59. The square wave signal from the multi-vibrator 57 is differentiated to provide pulses of alternating positive and negative polarity to the buffer amplifier 59. The buffer amplifier 59 is biased beyond "cutoff" by connecting the amplifier's control grid 112 to a negative voltage source 115 for blocking or suppression of pulses with negative polarity and delivers pulses in dependence on positive fluctuations of the signal from the multi-vibrator 57 to the common input circuit of the bistable multi-vibrator 58, which likewise in the usual way, while producing a square wave signal to be supplied to the coding apparatus 12 with the amplitude variations of the control signal, responds to the successively applied pulses from the buffer amplifier 59.

In order to facilitate the detailed explanation of the operation of the transmitter with respect to the manner in which the cyclic operation of the multi-vibrators 57 and 58 is interrupted or altered under the control of the coding signal generator 29 and the transposition mechanism, idealized signal waveforms, appearing in different parts of the transmitter, indicated by circled reference letters, given corresponding letter designations in the graphic illustration in fig. 4 and 5. For the time being, the effect of the second series of multi-vibrators 57' and 58', the delay line 71 and the coincidence circuit 46 shall be left out of consideration; as a result, the marking of the letter designations on the circuit diagram must also be disregarded. Curve A illustrates line drive pulses of positive polarity supplied from generator 19 to 5:1 blocking oscillator 30, where they are split to produce a pulse of the opposite polarity at the output channels of the oscillator for every fifth line drive pulse applied, as shown by the curve B. The output signal from the blocking oscillator 30 is continuously supplied to the control electrodes 104 and 109 in the multi-vibrator 57, during each sub-frame reflow interval, as well as each sub-frame reflow interval, while only pulses are supplied to the multi-vibrators 57 and 58 over conductors 41-44 during each frame-to-backrace interval.

During each single subframe rewind interval, the coding signal generator 29 may produce a number of coding signal pulses, as shown by curve C. It will be noted that the various pulses, shown in curve C, are so grouped or spaced as a function of time that there is a space or time interval between second and third pulses so that a coding signal pulse does not occur synchronously with a periodically returning pulse from the blocking oscillator in 30 (curve B) for a longer time. A coding arrangement to ensure such a distribution of the coding signal pulses in order to prevent conflict, which can arise from the simultaneous supply of pulses from the blocking oscillator and pulses from the generator 29 to the multi-vibrator 57, as indicated below, is described in more detail in detail in connection with fig. 11.

Pulses are supplied to the multi-vibrator 58 through the buffer amplifier 59, as well as from the generator 29 through a selected filter rectifier 21-26, but do not conflict with each other. Each pulse from the multivibrator 57 is preceded by a pulse from the blocking oscillator 30 or by a pulse from the generator 29, and the pulses from the generator 29 which reset the multi-vibrator 57 are not supplied to the multi-vibrator 58. As a result, the pulses emitted to the multi-vibrator 58 from the generator 29 occur at other times than the pulses, which are received from the multi-vibrator 57, and there can be no conflict. Of course, some of the pulses from the generator 29 can cause pulses to be supplied to the respective input circuits of both multi-vibrators 57 and 58. If this takes place, a suitable coupling can be arranged as a precaution to avoid any conflict in the system's function. If e.g. same pulse results in pulses being supplied to both multi-vibrators, a delay line can be arranged in the input circuit of one of them to avoid an unwanted ambiguity.

The particular coding signal combination or waveform C contains no more than one pulse of a specific frequency;

in practice, there may be cases of repeated pulse frequencies within code signal combinations, which are produced during some or all of the subframe flashback intervals.

The groups of code signals on the curve C are separated from each other and rectified in the circuit 21-26 for individual supply to the various input circuits of the transposition mechanism 27. This mechanism may provide any one of a plurality of circuit connections between its input circuits and the output conductors 31-35, so that the rectified pulses are applied to the normally closed gate circuits 36-40 in accordance with a coding scheme, which depends on the setting of the transposition mechanism. For illustration, it can be assumed that the mechanism 27 is set so that the filter rectifier 23 is connected to the conductor 31 for supplying len rectified signal group f„ on the curve D to the gate circuit 36, that the filter rectifier 24 is connected to the conductor 32 for supplying sel of the rectified f, group on the curve E to the gate circuit 37, that the f, filter rectifier 21 is connected to the conductor 33 for supplying the rectified signal group shown in the curve F to the gate circuit 38, that the f„ filter rectifier 26 is connected to the conductor 34 for supplying the rectified pulse on the curve G to the gate circuit 39. It shall also be assumed that the f» filter rectifier 22 above the mechanism 27 is connected to the conductor 35 for supplying the rectified f,.-signal group to the gate circuit 40, but then the effect of the coincidence circuit 46 will be taken out of consideration the moment the L signal group is suppressed. The filter rectifier 25 can of course also be connected to any of the output conductors 31-35 through the transposition mechanism, but since no f, signal group appears in the assumed code signal combination, this unit will not take any active part in the conversion and application of signal com -bination.

The normally closed gate circuits 36-39 likewise receive line drive pulses (wave form A) from the generator 19, and the signals on the curves D, E, F and G open for those of the line drive pulses, which occur simultaneously with the respective gate pulses. As a result, the signal on the curve H will be supplied over the conductor 41 to the control electrodes 122 and 123 in the multi-vibrator 58. The signal on the curve J is supplied over the conductor 42 to the control electrode 104 in the multi-vibrator 57 over the cross-connection filter from the anode 108, the signal on the curve K is supplied over the conductor 43 to the control electrode 122 in the multi-vibrator 58 over the cross-connected filter from the anode 125, and the signal on curve L is supplied over the conductor 44 to the control electrodes 104 and 109 in the multi-vibrator 57.

The pulses on curves B, L and J are thus applied to the multivibrator 57. For simplicity, this multivibrator is initially assumed to be in its first stable operating state, in which the discharge device 105 is non-conductive and the device 110 is conductive, as indicated by waveform M , which occurs at the anode 106, although the original operating condition is of no importance, where pulses with waveform B are supplied to the control electrodes for both tubes 105 and 110, thus causing blocking of the conducting tube, regardless of which one this is. Upon application of the first pulse of the form shown on curve B from the blocking oscillator 30, the discharge device 110 will therefore be made non-conductive, and by means of the well-known multi-vibrator effect, the tube 105 will become conductive. In a similar way, upon receipt of another pulse of the curve B, the multi-vibrator 57 will be triggered again, provided that the negative pulse is supplied to the control electrode 104 and makes the tube 105 non-conductive and the tube 110 conductive. As a result of the fact that the active pulses on the curve B are always supplied to the control electrode in both tubes of the multi-vibrator 57, this circuit is switched between its operating states by means of successive B pulses to produce the periodic rectangular wave, which shown by the first part of curve M, before the signal group from curve C has appeared.

In response to the f4 signal group from curve C, the pulse on curve J is applied to control electrode 104 in tube 105 across the cross-connected circuit from anode 108, and since the tube is now in its conducting state (as seen in curve M), the arriving active pulse cause disconnection of tube 105, while tube 110 becomes conductive. The pulse on curve B from the blocking oscillator 30, which occurs immediately after the f, signal group on curve C, is the next pulse, which is supplied to the multi-vibrator 57, and causes it to switch to the state in which the tube 105 is conductive and the tube 110 not leading. It will now be obvious that no signal group has been included in the combination on curve C between f, and f3 in order to avoid the possibility of the simultaneous supply of a pulse from the blocking oscillator and a pulse over conductor 44 or 42, which in the event would result in conflicting or opposite effect and lead to ambiguity or instability in the coding process.

The next pulse applied to multi-vibrator 57 is produced by the f„ group, which results in the pulse from curve L being applied to both control electrodes 104 and 109 to switch multi-vibrator 57 to its state, as shown by curve M The last two amplitude fluctuations, shown in the waveform M, are due to the supply of the last two pulses from the curve B, coming from the blocking oscillator, and mean a resumption of the periodic action of the multivibrator 57, which follows the end of the state-determining interval , i.e. the interval in which groups of code signal components occur.

The signal produced at the anode 106, namely the curve M, is supplied to the differentiating circuits 113, 114 to produce the signal shown on the curve N. This is applied to the control electrode 112 in the buffer amplifier 116, which is normally kept off by means of the source 115 ; as a result, only the differentiated pulses on curve N, which have positive polarity, will be converted through the buffer stage, accompanied by 180 degree phase inversion. The signal on the curve P therefore appears at the anode 117 in the tube 116, and is applied to the control electrodes 122 and 123 in the multi-vibrator 58 through the capacitor 118, respectively

119 for switching the multi-vibrator between its two operating states in the same manner as explained in connection with the multi-vibrator 57. During the partial image sweep intervals the pulses of waveform P correspond to alternating pulses from the curve B, and the frequency of the multi-vibrator 58 is therefore half of the frequency of the multi-vibrator 57. During the sub-frame flashback intervals or the state-determining intervals, the periodic action of the multi-vibrator 58 is interrupted or changed as a function of the coding scheme.

The negative pulses shown on curves P, H and K are then applied to multi-vibrator 58. This multi-vibrator is initially assumed to be in the steady state of operation in which discharge tube 120 is conductive and tube 124 is non-conductive , as indicated by the waveform Q appearing at the anode 125, although the initial operating condition is irrelevant as is the case with multi-vibrator 57. The first three pulses shown on curve P switch multi-vibrator 58 between its two stable operating conditions, as they are supplied to the control electrodes of the two tubes of the vibrator producing a periodic rectangular wave during each sub-frame sweep interval, as shown by the first part of the curve Q. In response to the 1, signal group, as shown by curve C, the pulse, as shown on curve K, is supplied to the control electrode 122 in the tube 120, across the cross-connected circuit from the anode 125, and since this tube is already in its non-conducting state (as it will ees of the curve Q), the supplied negative pulse has no effect. As a result, the waveform Q will not be subject to any amplitude deviation at this time. On receipt of the f,, signal group, the pulse as shown on curve H is supplied to both control electrodes 122 and 123, and as a result switches the multi-vibrator 58 to its other operating state. The other two amplitude deviations on the curve Q are due to the influence of the last two pulses from the curve P on the multi-vibrator 58.

Thus, if we consider only the action of the multi-vibrators 57 and 58, the signal, as shown on the curve Q, is supplied to the encoder 12 to serve as a control signal. The amplitude fluctuations in this signal adjust the beam in the coding apparatus 12, and since such fluctuations occur arbitrarily during the partial image rewind intervals, a very effective distortion of the image is achieved.

According to the present invention, the complexity of this coding process is significantly increased, whereby a system with a significantly increased degree of secrecy is achieved. From a consideration of FIG. 1, it will appear that the pulses on the curves H—L, which are supplied to the various input circuits for the bistable multi-vibrators 57 and 58, are likewise supplied to the various input circuits for the bistable multi-vibrators 57' and 58', but in a different way or sequence. The pulse from the curve H is supplied to the anode 108' of the multi-vibrator 5T, the pulse from the curve J is supplied to the control electrodes 122' and 123' of the multi-vibrator 58', the pulse from the curve K is supplied to the anode 125' of the multi-vibrator 58' and the pulse from the curve L is supplied to the control electrodes 104' and 109' in the multi-vibrator 57'.

The pulses on the curve B and those on the curves L and H are consequently supplied to the multi-vibrator 57', which works in a manner similar to that explained in connection with the multi-vibrator 57. If, therefore, e.g. it is assumed that the multi-vibrator 57' is initially also in its first stable operating state, in which the tube 105' is non-conductive and 110' is conductive, successively applied pulses of the curve B produce the rectangular waveform of the curve R at the anode 106', which is identical to the waveform of curve M before the arrival of the code signal groups from curve C. The first pulse applied to the multi-vibrator 57' apart from those from curve B is produced by the f„ signal group of curve C. Upon receipt of this signal group is supplied with the pulse from the curve H to the control electrode 104' above the cross-connected filter from the anode 108', but as the tube 105' is already blocked (as can be seen from the curve R), this supplied negative pulse has no effect. As can be seen from this, no amplitude change of the curve R takes place at this time. The next pulse applied to the multi-vibrator 57' is produced by the f„ signal group which results in the application of the pulse from the curve L to both control electrodes 104' and 109', and this switches the multi-vibrator 57', to its second operating state, as shown by the curve R. The last two amplitude fluctuations on the curve R are due to the supply of the last two pulses from the curve B to the multi-vibrator 57'.

The signal produced at the anode 106', namely that shown on the curve R, is supplied to the differentiating filter 113', 114'

for producing the signal on curve S. This signal is supplied to the control electrode

112' in the buffer amplifier 116', as normal

is kept blocked, so that only the differentiated pulses on the curve S with positive polarity escape through the buffer stage with

180 degree phase inversion. The signal, which is

shown on the curve T, therefore occurs at the anode 117' in the tube 116' and is supplied to the control electrodes 112' and 123' in the multi-vibrator 58' above the capacitor 118', respectively

119' for switching this multi-vibrator between its two stable operating states.

The pulses shown on the curves T, J and K are thus supplied to the multi-vibrator 58'. As is the case with multi-vibrator 58, multi-vibrator 58' is initially assumed to be in the operating state in which tube 124' is non-conductive and tube 120' is conductive, as shown by waveform U, which occurs at the anode 125' in the tube 124'. The multi-vibrator 58' therefore operates under the influence of successive applied pulses, as shown on the curve T to produce the rectangular waveform U, which is identical to that produced in the multi-vibrator 58, before any signal group from the curve C appears .

In addition to the pulses from the curve T, pulses from the curves J to K are also supplied to the multi-vibrator 58' for influencing it. In response to the f4 group, which causes the pulse from the curve J to be supplied to the control electrodes 122' and 123', the multi-vibrator 58' is switched from one stable operating state to the other with the consequence that at this time an amplitude change takes place on the curve U. Since the multi-vibrator 58' is already in its reference state (in which the tube 124 is conducting and the tube 120 is not conducting), the supply of the negative pulse on the curve K to the control electrode 122' across the cross-connected filter from the anode 125' has no effect. As a result, curve U undergoes no amplitude change until the receipt of the next pulse from curve T, which switches the multi-vibrator to its opposite state. The last amplitude fluctuation on curve U is caused by the last pulse on curve T.

A comparison of the output signal produced in the multi-vibrator 58'

(curve U) with the output signal from the multi-vibrator 58 (curve Q) shows some differences when code signals appear as shown on curve C and immediately after, which is due to the different circuit connections from gate circuits 36-39.

The output signal on the curve U is applied to the delay line 71, where it is slightly delayed to form the signal on the curve X, which in turn is applied to an input circuit for the coincidence circuit 46 to serve as an opening signal for it. The purpose of the delay line 71 will be described later. Meanwhile, the f. signal group on the curve C is rectified in the rectifier 22 to form the pulse on the curve V, which is supplied through the transposition mechanism 27 to the conductor 35 for supply to the normally closed gate circuit 40. This gate circuit likewise receives line drive pulses (curve A), and works under the simultaneous action of pulses from the curve V and line drive pulses from the curve A, which act simultaneously to supply a positive pulse of the curve W to the coincidence circuit 46. The coincidence circuit 46 transmits pulses or code signal components similar to those shown on the curve W to the conductor 47 for input to the blocking oscillator 30 only during certain discrete operating intervals, which are determined by the multi-vibrator 58'. In the particular case shown, the coincidence circuit 46 only allows the pulse on the curve W to pass, if the opening signal for the curve X is at this time at its upper or positive level. For the particular case illustrated, the amplitude characteristic of the curve X at the time the positive pulse from the curve W occurs is such that this pulse is passed on to the blocking oscillator 30 to reset it to its reference or O-count position.

In fig. 5, the curves B', M', N', P' and Q' illustrate the modifying effect of the multi-vibrators 57' and 58', the delay line 71 and the signal transfer circuit 46 of the present invention on waveforms, as illustrated by the respective curves B, M, N, P and Q. The application of the pulse as shown by curve W to the blocking oscillator 30 causes an accelerated action thereof, immediately interrupting its 5:1 count, as indicated] by pulse 69 on curve B' which occurs simultaneously and the pulse as shown on the curve W. The multi-vibrator 57 now works under the influence of the pulses from the curve B' in addition to those shown on the curves L and J. As a result, the multi-vibrator changes to its opposite state of the pulse 69 on the curve B', as shown with the curve M', which occurs at the anode 106 in the tube 105. Then the next pulse which is supplied to the multi-vibrator 57 is the one shown on the curve L and is supplied to both tubes 105 and 110, the multi-vibrator is reset to its opposite state, as shown at curve M'. The last two amplitude changes on the curve M' are caused by the last two pulses on the curve B'.

In a similar manner as explained above, the output signal from the multi-vibrator 57 is differentiated, as shown by the curve M' to form the signal on the curve N', and the positive differentiated pulses are applied with reverse polarity, as shown by the curve P' to the control electrodes 122 and 123 in tubes 120 and 124 respectively in the multi-vibrator 58. This multi-vibrator now works under the influence of the pulses from the curve P' in addition to those from the curves H and K. As a result, the multi-vibrator 58 switches to its opposite state depending on the pulse on the curve P', which occurs simultaneously with the pulse 69 on the curve B' to produce an amplitude change of the signal produced at the output leads of the multi-vibrator, namely the wave illustrated by the curve Q'. This modified output signal or control signal Q' is the signal which is supplied to the coding apparatus 12 for influencing it to achieve a coding; a comparison between curve Q' and Q shows that the effect of applying a modifying signal (curve X) from the blocking oscillator 30 to add an encoding or switching signal component to the oscillator has a very definite effect on the operation of the system. As will be apparent from the following, the receiver corresponding to the multivibrator 57' and

58', if the code-determining elements

or switches for the transposition mechanism in the receiver are not set correctly,

do not produce the correct modifying signal for varying the transmission state in the coincidence circuit 46, and correct decoding cannot be achieved. If the modifying signal is wrong, the control signal produced by the multi-vibrator 58 will likewise be wrong, and cannot be corrected until the code-determining switches are set to exactly the desired combination. As in the previously used systems

moreover, decoding is only achieved if the code-determining elements are correctly set, but the unauthorized person's reception of the coded television broadcast by, for example, manipulation of the code-determining elements while simultaneously observing the appearing image is made extremely much more difficult in the system according to the present invention, where an incorrect setting results in there

an erroneous modifying signal is induced in addition to a lack of clarification of the image. In practice, it has been found that the [ time required for decoding when attempted even by skilled engineers and technicians has increased to such an extent that such attempts are practically hopeless.

Another advantage, which is achieved by the use of a modifying signal according to the invention, is that the number of usable code signal combinations is significantly increased, compared to what can be used with the previous systems. There is actually no limitation either with the choice or composition of each code combination or group; all possible combinations can be used arbitrarily without restriction. To explain this, e.g. that the coincidence circuit and the modifying signal, which controls the circuit's functions, have not been used and that none of the code signal combinations, such as e.g. that shown at curve C comprises signal groups, which effect the transmission of pulses for resetting the input circuits 42 and 43, i.e. the input circuits which switch the multivibrators 57 and 58 to predetermined and precise operating states, separate from the common input circuits 44 and 41, which is used to switch each multi-vibrator from its instantaneous state, whatever this may be, to its opposite state. If one of the control devices (comprising circuits corresponding to 30, 57 and 58, as will be explained later) in one of the receivers should fail, due to the influence of noise or some other unwanted signal, it will not maintain synchronous operation with the transmitter. This is because all pulses in the code combinations are effective for switching the multi-vibrators (corresponding to 57 and 58) to their opposite states, provided that these pulses are not applied to the common input circuits, and no pulses are available for switching to specific operating states, which would effectively set the multi-vibrators in reference states in time with the transmitter. As a result, the usable code signal combinations for maintaining synchronism between the transmitter and the receivers in previous systems are limited, provided that some of the code signal groups must be used for setting the Input Circuits in order to set the system to certain reference states at certain intervals, as a precaution against effect of possible noise or other signals.

If, on the other hand, a modifying signal and a coincidence circuit corresponding to 46 according to the present invention are used, the system, in contrast to previous systems, will automatically fall back in time, even if the system can momentarily be thrown out of synchronism as a result of noise signals or other reasons. This is because the circuit 46 will sometimes cause the transmission of a pulse to the blocking oscillator, corresponding to 30, when it should not do so, namely when a corresponding pulse is not transmitted in the transmitter or, conversely, does not transmit a pulse when it should, all due to of the modifying signal being out of step or phase in the receiver. When this occurs, the function or non-function of the corresponding blocking oscillator will at some point effectively set the corresponding multi-vibrators in the same operating state as the multi-vibrators 57 and 58 in the transmitter, even though all pulses in the code combinations are routed to the common input circuits. When this occurs, the receiver falls back in step with the transmitter. It is clear that this feature allows a much wider range of choice of code-signal combinations, as it is no longer necessary to receive pulses in each combination, which cause switching of the multi-vibrators to their reference positions. As a result, there is no restriction at all in the choice or set-up of the code groups. This achieves a significantly increased degree of secrecy.

The effect of introducing a reset or reset signal component to the blocking oscillator 30 to form the modified signal in the waveform B' is also reflected in the operation of the multi-vibrators 57' and 58', as these multi-vibrators are likewise switched by the output signal from the blocking oscillator. In fig. 5 the modified waveforms R', S', T', TJ' and X' are therefore drawn, corresponding to R, S, T, TJ and X to illustrate how these signals are varied. Accordingly, the pulses shown on curve B' are applied to multivibrator 57' in addition to the pulses shown on curves L and H. As will be seen from curve R', which shows pulses occurring at anode 106' of tube 105 ', the multi-vibrator 57' is triggered by the pulse 69 to switch to its opposite operating state. This multi-vibrator is switched again by the pulse on the curve L, which is supplied to both tubes 105' and 110'; the last two amplitude fluctuations on the curve R' are caused by the supply of the last two pulses, which are shown on the curve B', and come from the blocking oscillator 30. The differentiated pulses, which are shown on the curve S' and the negative pulses on the curve T' is produced as explained above by the pulses on the curve R', and the signal on the curve U' appears at the anode 125' in the tube 124' in dependence on the negative pulses on the curve T, and also the negative pulses on the curves J and K. The signal on the curve U' is delayed in the delay line 71 to produce the signal on the curve X', which is the signal that is supplied to the coincidence circuit 46. As a result of the modifying signal, which is produced in the multi-vibrator 58', is used to produce pulses which can be effective in changing this modifying signal, the delay line 71 is arranged to introduce a slight delay between the time at which the modifying signal is generated and the time at which it is used. Therefore, although the modifying signal produced in the multi-vibrator 58' is provided at its positive level at the instant a line drive pulse such as that shown on curve W is applied to the coincidence circuit 46, this modifying signal, as a result of the delay introduced by means of the delay line 71, is not changed to its negative level by the impact of this pulse and thereby render the gate 46 ineffective until after the entire pulse has been introduced. In practice, the delay line 71 is not essential, and can therefore generally be omitted; warping the transmitted pulses after their leading edge is not in any way harmful, and the system works perfectly without the delay, although the gate signal itself can sometimes be modified by a transmitted trigger pulse immediately after the gate signal's leading or leading edge occurs.

Briefly stated, the subscription television system illustrated in fig. 1, a coding apparatus for changing the operating state of the system in accordance with a predetermined coding scheme. This apparatus comprises the encoder 12, the 5:1 blocking oscillator 30, the bistable multi-vibrators 57, 58, the buffer amplifier 59, the gate circuits 36-39, the transposition mechanism 27, the filter rectifier units 21-26 and the code signal generator 29. The system has a source of code signal components which can are regarded as pulses similar to those illustrated on the curve W, which occur in the conductor 45 every time a signal occurs in the frequency f0. Non-linear signal transmission means, which is constituted by the coincidence circuit 46, has a plurality of transmission states, and connects this source of coding signal components to the coding apparatus. More particularly, it connects the source to the blocking oscillator 30. Finally, the television system comprises means, such as the bistable multi-vibrators 57' and 58', the connecting line 71 and the buffer amplifier 59', which are connected to the coding apparatus, namely to the output of the blocking oscillator 30 and to the signal transmission or the switching device 46, for changing their transmission state in accordance with a predetermined scheme. In fig. 1, the transmission state of the coincidence circuit 46, as explained, is varied in accordance with a scheme which is substantially different from the coding scheme represented by the control signal produced in the multi-vibrator 58; however, the two coding schemes are adapted to each other by means of the common blocking oscillator 30, from which the two schemes are derived separately.

In order for a subscriber to be able to receive the coded broadcast, it is necessary that the combination of code signal trains, as shown on curve C, be made known to the subscription recipients. For this purpose, the coding signals are supplied to the mixing amplifier 13 over the conductor 66 to be combined with the composite video signal for transmission to subscription recipients. The signals with different frequencies appear individually between successive line drive pulses, which are superimposed on the blanking level of the vertical or partial image return signals, and in order not to disturb the subscriber's scanning system, it is desirable that the amplitude level of the return pulses be modified to effect an internal modulation of the code signals . To this end, pulses are supplied to the synchronization generator 19 over the conductor 28 to produce suitable modulating pulses, which in turn are supplied to the mixing amplifier 13 over the conductor 20 to code modulate the vertical pulses at suitable times.

It is obvious that the use of the video carrier wave for transmitting the coding information is not essential for the present invention, and that this information can be distributed in whole or in part in another way, e.g. d form of an auxiliary modulation of the sound carrier wave or of a special carrier wave or by means of wiring circuits that connect the transmitter with the subscription receivers. According to another alternative, such code information can be generated locally, e.g. by using perforated code cards, which are only issued to subscribers.

It should be mentioned at this point that in the illustrated embodiment, the code signals are produced during part of the vertical return interval, so that the interruption in the cyclic operation of the oscillator 30 and the multi-vibrators 57 and 58 is effected between sub-image pass-through intervals. However, it should be understood that the code signals can be generated and used during the partial picture pass-through intervals, especially in the event that the code information is distributed to subscribers according to one of the above-mentioned alternative methods.

It should be expressly emphasized that the code signals used for resetting or resetting the blocking oscillator, or the code signals used for influencing the multi-vibrators 57 and 58, may appear at non-uniform or arbitrary intervals, which may be longer or shorter than a sub-frame pass-through interval. If the total count ratio of the counting circuits (5:1 blocking oscillator, multi-vibrators 57 and 58) is incommensurable with the number of lines in a picture interval, the coded picture will produce an impression or illusion of moving or is scrolled towards the top or bottom of the picture screen, regardless of how often the code signals change the operating state. This effect occurs as a result of a code arrangement that produces state changes at the end of a sequence of operating steps, when the number of operating steps is not an equal multiple of the number of line intervals per image, will assume a different operating state at the beginning of successive images, so that the state changes do not occur with corresponding line runs in the successive images. This "wandering" effect occurs in the described system according to the present invention under the assumed operating conditions, as long as the influence by means of the code signal takes place less than once per frame interval, as the blocking oscillator 30 and multivibrators 57 and 58 need twenty line pass intervals to perform a normal complete duty cycle, and this number does not add up to a whole number of times the number of line passes in a frame interval (525 by current U.S. standard).

The receiver in fig. 6, which can receive the broadcast from the transmitter in fig. 1, comprises a radio frequency amplifier 130, with input terminals which are connected to an antenna 131 and an output circuit which is connected to a first detector 133. The first detector is connected through an intermediate frequency amplifier 134 to another detector 135, which in turn is connected to a video amplifier 136. The video amplifier is connected via a decoding device 140 to the input terminals of a cathode ray imaging device 141. The decoding device 140 can be implemented in a similar way to the coding device 12 in the transmitter with the exception that it is designed to to work complementarily to effectively compensate for the variations in the time relationship between the video component and the synchronization component in the received television signal. A complementary function of the decoding device can be ensured by reversing the anode connections in the beam deflection tube compared to the anode connections used in the encoder on the transmitter side.

The second detector 135 is also connected to a synchronization signal separator 142, the output circuit of which is connected to an image sweep system 143 and a line sweep system 144. These sweep systems are connected to suitable deflection elements in the reproduction device 141. In order to facilitate the separation of the code signals from the composite television signal , it is desirable to arrange a circuit for selecting or displaying only the part of the composite video signal which contains the aforementioned code signals. To this end, image drive pulses are diverted from the synchronization signal separator 142 and supplied to a monostable multivibrator 160, which has its output circuit connected to a normally open gate circuit 159. The output circuit of the video amplifier' 136 is likewise connected via the conductor 191 to the gate circuit 159 for supplying the composite video signal to this, and the output circuit of this gate circuit is connected to a series of filter rectifiers 21-26, which are identical to correspondingly numbered rectifiers in the transmitter. Line drive pulses are taken from the sweep system 144, and supplied to a blocking oscillator 30, which is likewise identical to the correspondingly numbered oscillator ten transmitter. As indicated by the use of identical reference numbers, the other part of the connection diagram in fig. 6 identical to the corresponding elements in the transmitter in fig. 1.

When using the receiver in fig. 6, the encoded f iron vision signal is received from the transmitter in fig. 1 of the antenna 131, is amplified by the radio frequency amplifier 130, and is converted by heterodyne action to the selected intermediate frequency for the amplifier in the first detector 133. The resulting intermediate frequency signal is amplified in the intermediate frequency amplifier 134, and likewise in the second detector 135 to produce a composite video signal. This last signal is amplified in the video amplifier 136, and transmitted through the decoding device 140 to the input terminals of the image reproduction apparatus 141 for controlling the intensity of the electron beam in a known manner.

The synchronization components are separated in the separator 142, the synchronization component being used for synchronization of the sweep system 143 and thus the partial image deflection in the image reproduction apparatus 141, while the line synchronization components are used for synchronization of the sweep system 144 and thereby the line deflection in the apparatus 141. Of course, the audio modulated carrier wave is received, which arrives with the video carrier and is reproduced in a suitable audio system, which is omitted from the drawing for simplicity.

Decoding in the receiver is achieved in a completely similar way as explained above in connection with coding in the transmitter; the waveform in fig. 4 and 5 are therefore illustrative of the receiver as well as of the transmitter, and corresponding letter designations therefore also refer to the receiver diagram in fig. 6 to the extent appropriate. Of course, the transposition mechanism in the receiver must be set to the same setting as that used in transmitters, and an appropriate fee calculated for the switch setting information.

The receiver in fig. 6 has a feature which has no counterpart in the transmitter in fig. 1. This feature consists in a mixer 161 which is connected to the output terminals of the 1 multivibrator 160 for extracting the opening pulses used for introducing the code signals. The mixer 161 is also connected to a circuit 162 which receives line drive pulses from the sweep system 144. The mixer 161 has its output terminals connected to the input terminals of the image reproduction device 141, and the purpose of this mixer is to supply a signal during the partial image return as well as during the partial image pass-through intervals, thereby ensure complete blackout of the image during these intervals. This feature is desirable due to the code modulation of the code signals, as described in connection with the transmitter. Furthermore, switching off is desirable during the line return provided that the switching effect of the decoding apparatus 140 is provided during selected line return intervals, and swing-in pulses, which are produced at this time, would otherwise be able to supply current to the electron beam in the apparatus 141.

According to another embodiment of the invention, another set or series of bistable multi-vibrators 57' and 58' can be eliminated together with the buffer amplifier 59', and the same control signal produced in the multi-vibrator 58 for influencing the coding device 12 can itself be used as a modulating or opening signal for supply to the coincidence circuit 46. Fig. 7 illustrates how the transmitter in fig. 1 and the receiver in fig. 6 can be changed so that this is achieved. This arrangement has the obvious advantage that approximately the same result is achieved with a significantly reduced number of elements. In the embodiment according to fig. 7 code signal components appearing in the conductor 45 are supplied to the gate circuit or the coincidence circuit 46, and if these pulses occur during time intervals when the control signal, which is developed in the bistable multi-vibrator 58, acquires a predetermined value, in the case shown positive, these pulses will be transmitted to the blocking oscillator 30, to set it in a reference state or O-count state.

As is the case with the embodiment according to fig. 1, a plurality of code signal components are produced in the gate circuit 40, which act in the conductor 45. A control effect is obtained at the output of the coincidence circuit 46, as a reaction to some of the code signal components, and the television signal is coded or decoded at least partially in accordance with this control effect. The control effect takes the form of a modifying signal, which is supplied from the encoding or decoding apparatus (from the bistable multi-vibrator 58 and not from the oscillator 30 of Fig. 1), and represents a selected scheme; certain of the coding components are selected in dependence on the modifying signal by the coincidence circuit 46. According to fig.

1, the selected scheme for the modifying signal, as it is finally supplied to the coincidence circuit 46, will be substantially different from the scheme for the modifying signal, used according to fig. 7 as a result of the modifying signal according to fig. 1 is written from the output of the blocking oscillator 30 and is significantly modified by the multi-vibrators 57' and 58'.

A further variant of the invention is illustrated in fig. 8, according to which they in fig. 1, 6 and 7 have been modified, so that the signal transmitting body or the gate circuit 46 is introduced in one of the input circuits of one of the bistable multi-vibrators, which are used for generating a control signal for the coding apparatus 12. In this case, the gate circuit 40 be directly connected to the blocking oscillator 30, so that every time a signal of frequency f, (in the assumed example) occurs, the blocking oscillator 30 is reset to its reference or O-count position. The coincidence circuit 46 is now placed between one of the gate circuits, which e.g. the gate circuit 37 and the reset input circuit 42 of the bistable multi-vibrator 57 which is connected to the control electrode 104 of the device 105 across the cross-connected circuit from the anode 108. The input conductor 44 may remain unchanged.

Every time an f, signal by this device occurs, a line drive pulse will be let in and supplied to one input circuit for the coincidence circuit 46, but may or may not be passed on to the conductor 42 for impingement on the multi-vibrator 57 depending on the instantaneous amplitude characteristic of the modifying signal, which appears at the output of the bistable multi-vibrator 58, and is fed back to the coincidence circuit 46. Although the modifying signal admits a pulse through the coincidence circuit 46 to the control electrode 104 of the tube 105 in the multi-vibrator 57, this pulse will still be without effect, as the multi-vibrator may already be in its stable operating state, in which tube 105 is closed and tube 110 is conductive.

Yet another embodiment of the invention is illustrated in fig. 9. The connection illustrated in this figure can be used in the transmitter in fig. 1 and in the receiver in fig. 6 to achieve that the modifying signal can be derived from an element other than the device used to influence the coding or decoding apparatus. In the previously described embodiments, the blocking oscillator 30 comprises the multi-vibrators 57 and 58, the buffer amplifier 59, the gate circuits 36-39, the transposition mechanism 27, the rectifiers 21-26 and the key signal generators 29 a control device for producing a control signal at the output terminals of the multi-vibrator 58, with an amplitude characteristic varying between at least two predetermined values or levels in accordance with a predetermined coding scheme. In the embodiment according to fig. 1 and 6 is a modifying apparatus comprising the multi-vibrator 57' and 58' connected to this control device, in particular to the output terminals of the blocking oscillator 30 for producing a signal which is converted into a modifying signal, which likewise has an amplitude characteristic which varies between at least two predetermined values in accordance with a control scheme, which in turn is used to transfer code signal components to the control device or more specifically to an input circuit of the blocking oscillator 30, only during separate time intervals, when the amplitude characteristic has a predetermined value. In the embodiment according to fig. 7 and 8, the control signal produced in the control device and supplied to the coding or decoding apparatus is used, itself as a modifying signal, to effect the transfer of certain coding signal components to the control device.

In the embodiment according to fig. 9, the modifying signal is taken from the encoding or decoding apparatus itself instead of from the control device which affects the encoding or decoding apparatus. The encoding apparatus 12 and the decoding apparatus 140 have already been described as preferably comprising beam deflection tubes with electron beams which are oscillated back and forth between two segment anodes. A delay line is arranged in the circuit between a target anode and the input circuit of the mixing amplifier 13 or the image reproducing apparatus 141 depending on whether it is an encoding or decoding apparatus, and the other target anode is connected directly to the mixer 13 or the image reproducing apparatus 141. A circuitry may be connected to one or other of these target anodes for deriving the video signal only during one mode of operation or operating condition, and this intermittent video signal may be filtered to produce an irregularly shaped signal with amplitude fluctuations, representing the coding scheme of the control signal, which is supplied to the coding device for initiation of the coding operation. This irregularly shaped signal can be applied to a bistable multi-vibrator 48 to vary it somewhat (namely to eliminate any other amplitude variation), although this is not necessary before the signal is applied to the coincidence circuit 46 for opening purposes. The gate circuit or coincidence circuit 46 operates as described above, and transfers pulses to the blocking oscillator 30 depending on the simultaneity or coincidence of pulses from the gate circuit 40 and positive pulse components of the opening signal supplied from the multi-vibrator 48.

In the embodiment according to fig. 10, the transmission or switching device has the form of a selector instead of a coincidence circuit or gate circuit, so that essentially all the code signal components, which appear at the conductor 45, are used in the modifying function of the coding scheme. For example can a selector 61, which can be an ordinary electronic switch, comprise an intensity control grid, a pair of output anodes and a deflection control signal for switching the code signal modulated electron beam between the output anodes, provided with two output circuits, one of which above one conductor 62 is connected to the blocking oscillator 30 for switching, while the other over a conductor 63 is connected to the switching input circuit 42 of the bistable multi-vibrator 57. The selector 61 has two input circuits, one of which is connected across the conductor 45 with the gate circuit 40 and the other is connected to the output terminals of the multi-vibrator 58. When the amplitude characteristic of the control signal produced in the multi-vibrator 58 is established at a level, the selector 61 is set in an operating or transfer state for supplying pulses from the conductor 45 to the conductor 62 and thence to the blocking oscillator 30. When, on the other hand, the amplitude characteristic is established on the other nine 5, the selector 61 is switched to its second operating state, in which pulses are supplied from the conductor 45 to the conductor 63, and from there to the bistable multi-vibrator 57. In the embodiment according to fig. 10, essentially all code signal components from the gate circuit 40 are used to modify the coding schemes. It should of course be noted that as long as the conductor 63 is connected to the switching input circuit for the multi-vibrator 57, some of the supplied pulses from the selector 61 will have no effect as a result of the tube 105 already being in its non-conductive state.

As mentioned above, it receives the aid

bile multi-vibrator 57 coding pulses during subframe rewind intervals in addition to pulses from the counting device or blocking oscillator 30. It is desirable to

interrupt the function of the blocking oscillator on the receiver side or of the synchronizing effect of the blocking oscillator on the transmitter side, so that pulses from this counting device do not appear simultaneously with any of the pulses from the code combinations to avoid possible ambiguity or instability resulting from the simultaneous supply of pulses from two sources. If the counters or blocking oscillators are disabled on the receiver side, an additional gate circuit is needed. On the other hand, if the output from the counting device and the code combinations are synchronized on the transmitter side, the output pulses of the counting device will always occur at corresponding times in each partial image return interval. This considerably limits the complexity of the image distortion.

According to a feature of the present invention, the periodically repeated components of the blocking oscillator are allowed to appear at different times during each subframe pass interval to increase the complexity of the coding while the only circuitry used to avoid interference between the periodically repeated components and the coding signal components is located on the transmitter side. This is achieved by the device shown in fig. 11. Line drive pulses, which are shown as a partial reproduction of curve A (on a reduced time scale) in fig. 12, is effectively split in the blocking oscillator 30 to produce the pulses on the curve B of fig. 12, which is supplied to the bistable multi-vibrator 57 in the device 134 to achieve a coding in the same way as previously described. To ensure that no pulse on curve B in fig. 12 acts simultaneously with a coding component, the pulses on curve B are also fed to a monostable multi-vibrator 131 to produce the positive pulses shown on curve Z. A gate circuit 133 is of the normally closed type, and opens in response to each positive pulse on the curve Z for supplying the signals from the signal generator 132, which is contained in the coding signal generator 29 in fig. 1 to the multi-vibrators in the device 134.

The combination on curve C is shown on a reduced time scale in fig. 12 to illustrate the manner in which only the signals occurring during the positive pulses of the curve Z are allowed to affect the multi-vibrator 57 of the device 134. It will be seen that there is a gap or time interval between the second and third signal wave trains on the curve C, fig. 12, as the gate circuit 133 is in its closed state when receiving signals from the signal generator 132, which can possibly occur during this time interval. The combination on the curve C is supplied to the multi-vibrator 57 in the device 134 together with the pulses on the curve B, and since no pulse on the curve C occurs simultaneously with the pulse 138 on the curve B, the coding apparatus in the device 134 is affected without any interference between supplied pulses. Of course, only the combination on the curve C is transmitted to the subscribers, and not the signals from the signal generator 132, either as an additional modulation component for the television signal or through a special channel such as a wire. In subscription receivers, the blocking oscillator 30 is affected synchronously, and since no pulse from the blocking oscillator occurs at the same time as any of the pulses in the code signal combination, interference or ambiguity in the function of the coding apparatus is effectively excluded.

It will thus be seen that the blocking oscillator 30 produces a number of periodically repeated signal components, and the signal generator 132 produces combinations of code signal components, which have variable phase in relation to the periodically repeated components, certain of the code components acting essentially simultaneously with the components of series from the blocking oscillator. It will further be understood that the monostable multi-vibrator 131 and the gate circuit 133 use the periodically repeated signal components to effectively remove these code signal components from the combinations. The device 134 uses the periodically repeated components from the blocking oscillator 30 and likewise the combinations in the code signal components from the generator 132 with the exception of the aforementioned components in order to code the television signal.

As previously mentioned, the blocking oscillator must be kept in tact, and for this purpose return or switching pulses are generally produced which are used on the transmitter side and on the receiver side at suitable intervals. In order to increase the complexity of the coding, it has been found desirable to reset or reset the counting device at random times, and in a manner known only to the subscription recipients. However, if a counter device, such as a blocking oscillator, is reset during an earlier stage of the complete duty cycle, unstable operation may occur. For example if a 5:1 blocking oscillator, which is affected by line drive pulses, is used, it has been found that any attempt to reset the blocking oscillator immediately after it has generated a pulse or during the first two stages of its 5-stage cycle can result in instability or inaccuracy reset. According to a further feature of the invention, the output pulses from the counting device are therefore used to control the effect of the randomly occurring reset pulses to ensure that no reset pulse is supplied to the counting device during certain parts of its working cycle.

A device for achieving this result is shown in fig. 13, where the output pulse from the counting device 30 is supplied not only to the multivibrator 57 in the device 134, but also to a monostable multi-vibrator 136, which, depending on each output pulse from the blocking oscillator 30, produces a negative pulse with a duration comprising two line-through intervals . These last pulses are supplied to a normally open gate circuit 137, which is supplied with reset pulses appearing at random intervals over a conductor 138 for closing the gate during the negative pulse intervals. The manager 138 can e.g. be connected to the control signal generator in the device 29 which produces the signals used for resetting the blocking oscillator 30, and since all the generators are each operated so that it emits signals at random intervals, the control used can be considered a source of switching or resetting pulses. If pulses then happen to arrive across conductor 138 during the first two line-through intervals following an output pulse from the blocking oscillator 30, i.e., during the first two stages of the 5-stage duty cycle of the blocking oscillator 30, this reset pulse will be ineffective with regard to reset, as it will not be allowed to pass through gate circuit 137.

Briefly stated, the blocking oscillator 30 forms a multi-stable counter, which responds to each series of applied pulses to perform a complete duty cycle to produce an output pulse. A number of pulses are supplied to the counting device 30 from the synchronization generator 19. The reset signal source (i.e. one of the generators in the device 29) generates reset pulses occurring at random intervals for returning or readjusting the counting device to a predetermined reference step. Finally, the multi-vibrator 136 and the gate circuit 137 enable the use of each output pulse from the counter 30 to disable any reset pulse from the source 138 occurring during certain of the operating steps.

The invention thus provides a subscription television system, in which a modifying signal is taken from the encoding or decoding apparatus, and in turn is used to vary the operation of the same encoding or decoding apparatus, thereby changing the coding scheme with which the television signal would otherwise be imprinted. Furthermore, the coding or decoding apparatus can be controlled simultaneously by code signal components and a number of periodically repeated signal components, which are produced by a multi-stage counting device, the two signals being synchronized so that the components do not interfere with each other. The multi-stage counter can be reset by means of randomly spaced pulses only during certain of the various stages of the counter's duty cycle.

Claims (8)

1. Subscription television system consumption either in a transmitter or in a receiver and comprising an encoding (decoding) device with a plurality of working states, each of which creates a different mode of operation in the transmitter (receiver), an encoding (decoding) generating in - l direction comprising a source for randomly occurring coding or decoding signal components associated with a plurality of adjustable coding (decoding) determining elements whose momentary setting determines at least partial imprinting of said signal components on a control device, as on side ] produces a control signal representing a predetermined secret code scheme, and a device for utilizing the control signal to switch the coding (decoding) device between its operating states to vary the mode of operation of the transmitter (receiver), character - i cert by a feedback between an ] output and an input of the control device (30, 57—59—58) via an oms tillingin- 1 direction (46 resp. 61) from the output of which a modification signal is applied to the switching device which is in relation to the aforementioned predetermined secret code scheme, and which is used to switch from time to time at least part of the control device to a predetermined reference state in order to switch the encoding (decoding) device in a manner determined by the predetermined secret coding scheme modified by the switching device. 2. System according to claim 1, characterized in that the switching device (46 or 61) has a plurality of transmission states and the feedback modification signal varies the transmission state of the switching device (46 or 61) only during certain mutually time-shifted work intervals. 3. System according to claim 1 or 2, characterized in that the switching device has the form of a gate circuit (46). 4. System according to claim 3, characterized in that the control device comprises a plurality of cascaded, non-linear signal transmission stages (30, 57—59—■ 58) to switch the encoding or decoding apparatus (12, 140) between its working states, as it encodes (decoding) - signal generating device (40) is connected to a predetermined one of the aforementioned signal transmitting stages (30). 5. System according to claim 4, characterized in that the feedback comprises a connection (57-59', 71, fig. 1) between the output of the aforementioned one signal transmitting stage (30) in the control device and the gate circuit (46) to carry the modification signal to this one. 6. System according to claim 4, characterized in that the feedback (Fig. 7) connects the output of a further signal-transmitting stage (58) in the control device with the gate circuit (46) which is inserted between the coding (decoding) signal-generating device (40) and said one signal transmitting step (30). 7. System according to claim 4, characterized in that the feedback (fig. i) connects the output of another signal-transmitting stage (58) in the control device with the gate circuit (46) which, in turn, is connected to a third signal-transmitting stage (57 ) in the control device between the former and the second stage (30, 58). 8. System according to claim 1, characterized in that the control device (30, 57—59—58) comprises a plurality of per se known input circuits and where the switching device has the form of a selection device (61, fig. 10) which is connected to an encoding (decoding) signal generating device (40) and to the aforementioned plurality of input circuits and which transmits the encoding or decoding signal components to some selected ones of the input circuits in the control device (30, 57—59—58), as the feedback connects the output stage (58) in the control device (30, 57—59—58) with the selection device and causes it to supply coding or decoding signal components to one of the said input circuits during certain mutually separated working intervals and to another of the said input circuits during other mutually separate work intervals.